This invention relates generally to refrigeration compressors, and more particularly to small hermetic refrigeration compressors used in household appliances such as refrigerators and food freezers.
Because of high energy costs and various governmental requirements, household appliances are being extensively redesigned to increase their energy efficiency. In the case of refrigerators and food freezers, substantial improvements have been made by using superior insulation and by various other improvements in the refrigeration system itself, including the sizing of evaporators and condensers. One of the areas that has received the most attention is to increase the efficiency of the refrigeration compressor itself. This efficiency is expressed as the energy efficiency ratio (EER), which is measured by dividing the BTU/hr. output by the wattage power consumption under standard running conditions, and the resulting ratio is the energy efficiency ratio where the higher the number, the greater the efficiency. At one time, certain refrigeration compressors may have had an energy efficiency ratio as low as 2 to 2.5, but current developments have increased this above 4, and some compressors have been known to approach the ratio of 5 or better.
Increases in compressor efficiency have come primarily from increases in the electrical efficiency of the motor and from increases in the volumetric efficiency of the pump. One area that has received little attention is the area of mechanical friction, because it has not been recognized that any suitable means could be used to decrease the bearing friction of the principal rotating part, namely, the crankshaft. The crankshaft on these compressors generally rotates about the vertical axis, and therefore requires journaling in radial bearings to position and confine its rotation about the axis and using a thrust bearing to take the weight of the crankshaft and the parts mounted thereon. These bearings have generally been of the plain or oil film type, using a hardened steel crankshaft running in machined bearing surfaces on a cast iron frame, and the thrust bearing has commonly been in the form of hardened flat washers which may either float or be non-rotatably secured to the crankshaft or other parts. Such fluid film bearings have offered quiet operation and long life, while providing an acceptable level of friction, and therefore efforts to utilize rolling element bearings have generally not been successful, not only because of the higher cost, but also because of increased noise and shortened life of operation of such bearings.
It is recognized that in larger size compressors, such as multiple piston types used for central air conditioning units, ball and roller type bearings have been used with considerable success, and in fact have become almost necessary because of some of the high bearing loads in such units. Such units have positive displacement oil pumps and oil circulation under pressure to provide adequate lubrication for the rolling element bearings, and generally the locations where such units are used do not make extremely quiet operation a necessity. However, efforts to utilize such rolling element bearings on small household-type compressors run into a scale factor problem, where on the much smaller units there is insufficient space for many rolling element bearings, and they represent a substantial increase in cost over the plain fluid film bearing. Furthermore, the application in household refrigerators requires an extremely low noise level to be acceptable, and such compressors are required to have a long life of 10 or more years, often operating under a better than 50% duty cycle. Furthermore, such household compressors, which normally operate using two-pole motors of a nominal speed of 3600 rpm, utilize radially extending surfaces and centrifugal force to produce the necessary oil circulation for lubrication, and such lubrication is not as readily controllable, nor as predictable, as the lubrication provided by a positive displacement pump such as is used in larger compressors.